Semiconductor devices are used in a variety of electronic applications, such as personal computers, cell phones, digital cameras, and other electronic equipment, as examples. There may be millions of transistors and capacitors formed on a single integrated circuit (IC), for example. Semiconductor devices are fabricated by depositing various material layers over a semiconductor wafer, such as conductors, semiconductors, and insulators, and patterning the various material layers to form circuit elements and interconnects therebetween.
The various material layers of semiconductor devices are typically patterned using lithography techniques to create components and circuits. In semiconductor lithography, a resist is deposited or spin-coated onto a wafer substrate, and is selectively exposed to a form of radiation, such as ultraviolet light, electrons, or x-rays, as examples. An exposure tool and mask are typically used to effect the desired selective exposure. Patterns in the resist are formed when the wafer undergoes a subsequent development step. The areas of resist remaining after development protect the substrate regions that they cover during subsequent processing. Locations from which resist has been removed can then be subjected to a variety of subtractive (e.g., etching) or additive (e.g., ion implantation) processes that transfer the pattern onto the substrate surface. The areas of the resist exposed to the energy are made either soluble or insoluble in a specific solvent known as a developer. In the case where the irradiated (exposed) regions are soluble, a positive image of the mask is produced in the resist, referred to as a positive resist. If the non-irradiated regions are dissolved by the developer, a negative image results in the resist, referred to as a negative resist.
There may be 20 or more masking layers used to manufacture an advanced integrated circuit, for example. The trend in semiconductor technology is towards scaling down the size of semiconductor devices, for increased speed and decreased power consumption, as examples. For smaller-scale semiconductor devices, resists are typically used that are exposed at 248 nm or 193 nm wavelengths. The term “actinic” refers to the wavelength of the radiant energy, especially in visible and ultraviolet regions of the light spectrum, by which photochemical changes are produced in a radiation sensitive resist.
A resist typically comprises three components: a resin or matrix material that functions as a binder; an active ingredient or photoactive compound (PAC); and a solvent, which maintains the resist material in a liquid state until it is applied to a semiconductor wafer. The photoactive component of a resist is the component of a resist material that undergoes a chemical reaction in response to actinic radiation.
Optical properties are fundamental to understanding the response of semiconductor materials to radiation, such as in the exposure of resists. Simulation and modeling require knowledge and characterization of optical properties of a film in order to predict processing results under various processing conditions. Therefore, measurement of optical properties of films used in semiconductor manufacturing is important in simulating the results that are obtainable with photolithography.
An important optical property that impacts patterning is the index of refraction of a resist material, as an example. It is therefore important to have the capability to measure optical properties such as the real component (RI) and imaginary component (k) of the refractive index at the actinic or exposure wavelengths of films formed on semiconductor wafers. Often the variation of the refractive index of a film across the surface of a wafer is of interest, so many measurements are taken at many locations across the wafer, e.g., 49 measurements may be made across a wafer.
A significant problem with measuring optical properties of radiation sensitive resist materials is that the measurement method usually requires that the material be probed with a beam of radiation at the wavelength for which the material is labile or unstable. Consequently, the accurate measurement of film properties at the actinic wavelength of a film can cause chemical and/or physical changes to the material. Portions of films deposited on semiconductor wafers may be exposed during the measurement and testing process, which prohibits the films from being usable as material layers or pattern-generating resists, as examples. Thus, what is needed in the art is a method and system for measuring optical properties, such as the refractive index, of films deposited on a semiconductor wafer, that do not cause chemical changes to the films.